The invention relates to mooring buoyant bodies, and more particularly to mooring said bodies to remove instabilities inherent in flotation.
There are many activities now being conducted in watercovered areas of the earth's surface. These areas include lakes, oceans, seas, rivers or other bodies of water, the terms for which can be used interchangeably for purposes of the invention. In the petroleum industry, for example, the search for new sources of oil and gas has been extended particularly to the world's oceans. The oceans and other bodies of water present numerous problems to such activities, the primary one of which is to develop buoyant bodies such as platforms as bases of operations. Due to wave action of the oceans, platform stability can be an important factor in some activities such as drilling oil and gas wells, in which movement of a drilling platform can bend or break drill pipes.
A number of types of offshore drilling platforms have been proposed. These include platforms supported by columns having their lower ends resting upon the ocean floor (U.S. Pat. No. 2,248,051 by Armstrong); floating elevated platforms supported by columns from a floating pontoon (U.S. Pat. Nos. 3,011,467 by Le Tourneau and 3,163,147 by Collipp); a permanent floating platform employing counterweights for adjustment to vertical oscillations and reduction of roll (U.S. Pat. No. 3,294,051 by Khelstovsky); and floating platforms provided with anchors with parallel anchor lines, which are also known as semi-submersible platforms and tension leg platforms (U.S. Pat. Nos. 2,399,611 by Armstrong, 3,154,039 by Knapp and 3,540,396 by Horton).
The prior platforms having columns resting on the ocean floor are stable but are limited to relatively shallow water, due to the expense involved in building tall support columns. This and related costs such as maintenance make such platforms uneconomical at water depths greater than 350 to 400 feet. The floating platforms are much less expensive than the columnar type, but they are subject to influence by wave action and are thus much more unstable. This is particularly true of the unanchored platforms but is nevertheless present to an undersirable degree in both the anchored and unanchored types disclosed by the prior art.
The platforms disclosed in Armstrong, Knapp and Horton are anchored, or moored, with cables in a parallelogram geometry in which all the cables extend parallel to each other to the ocean floor. The parallelogram geometry essentially eliminates rotational motions about axes in the plane of the platforms' horizontal surface. These rotational motions are of two types: pitch, which is rotation about an axis normal to the direction of ocean currents; and roll, which is rotation about an axis in the direction of the ocean currents. The parallelogram geometry does not, however, eliminate lateral, or translational motions of the platform. These translational motions include surge, which is movement in the direction of current flow, and sway, which is movement normal to the direction of current flow.
A second conventional geometry for mooring lines is catenary in which the platform is moored with cables anchored laterally from the normal parallelogram anchor positions. The catenary geometry is helpful in eliminating horizontal excursions of platform, but it inherently introduces rotational motions.
A combination of the two mooring gemoetries can be used to provide some of the advantages of both. The parallelogram geometry provides the primary restraint while the catenary provides the secondary. However, the catenary mooring is limited in the amount of restraint that it can provide without introducing rotational motions. Under extreme conditions the restraint can be so great as to cause the adjacent cable in the parallelogram configuration, known as a tension leg, to go slack and cause the platform to pitch or roll.
An object of the invention, therefore, is to provide improved means for eliminating rotational and translational motions from floating platforms or other bodies.